Fix calculation of ep_io_write_bytes

Previously it contained the full length of the "blob" which
would contain the flexible metadata part. This commit "corrects"
the calculation by setting it to the number of bytes in the
document plus the number of bytes in the metadata section
stored for the document.

Change-Id: I9ccc92009723fe243a87cd6ec07f24129fdfdff6
Reviewed-on: http://review.couchbase.org/37400
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>